One of the basic principles when deploying a cellular telecommunication system is the re-use of frequencies in order to increase the served area capacity. However, re-using the same frequencies in other cells of the system, introduces co-channel interference that results in capacity and coverage losses in the system. In a CDMA system, the same frequency is typically used in all cells. Hence, other means have to be applied in order to control the co-channel interference. Therefore, special channelisation codes are used to separate signals for different users, and to suppress the co-channel interference.
On the other hand, when a radio system is co-existing with a radio system operating on another carrier frequency, it will experience some amount of adjacent channel interference due to imperfections in receivers and transmitters. This additional interference will then result in capacity and/or coverage losses unless some kind of a countermeasure is applied.
A traditional way to combat the impact of any additional interference and to maintain a certain Signal-to-Noise Ratio (SNR) is to increase the transmitted signal power. This can be understood by e.g. looking at the equation for the SNR of a Wideband CDMA (WCDMA) downlink (i.e. the direction from the radio base station to the mobile terminal) traffic channel between user i and radio access port n. A radio access port within a radio base station may comprise a baseband part 102, a modem 104 and a transceiver 106 The radio access port may also e.g. comprise other parts or a plurality of transceivers. However, in this invention the radio access part comprises at least one transceiver. Thus,
      SNR          i      ,      n        =            W              R        i              ·                  P                  i          ,          n                                      L                      i            ,            n                          ⁡                  (                                    (                                                ∑                                      b                    =                    1                                    B                                ⁢                                                      P                                          tot                      ,                      b                                                                            L                                          i                      ,                      b                                                                                  )                        -                                          (                                  1                  -                                      α                    i                                                  )                            ⁢                                                P                                      tot                    ,                    n                                                                    L                                      i                    ,                    n                                                                        +                          I                              ext                ,                i                                      +                          N              i                                )                                    where        W is the chip rate [s−1]        Ri is the data rate for the traffic channel [bit/s]        Pi,n is the transmitted power for the traffic channel [W]        Li,n is the path loss between user i and radio access port n        B is the number of radio access ports in the system        Ptot,b is the total output power for radio access port b [W]        αi is the downlink orthogonality factor for user i        Iext,i is the received external interference for user i [W]        Ni is the receiver noise floor [W]        
Thus, it can be stated that the required total transmitted signal power required to serve a certain amount of users depends among other things on the level of the inter-cell and inter-system interference.
On the downlink in a WCDMA system, a number of physical common channels are used to transmit control information, e.g. the pilot channel, the broadcast channel, the synchronization channel and the paging channel, over the whole cell area. The SNR experienced by user i for one such common control channel can be calculated as
      SNR          CCH      ,      i      ,      n        =            W              R        CCH              ·                  P                  CCH          ,          n                                      L                      i            ,            n                          ⁡                  (                                    (                                                ∑                                      b                    =                    1                                    B                                ⁢                                                      P                                          tot                      ,                      b                                                                            L                                          i                      ,                      b                                                                                  )                        -                                          (                                  1                  -                                      α                    i                                                  )                            ⁢                                                P                                      tot                    ,                    n                                                                    L                                      i                    ,                    n                                                                        +                          I                              ext                ,                i                                      +                          N              i                                )                    where                W is the chip rate [s−1]        RCCH is the data rate for the common channel [bit/s]        PCCH,n is the transmit power for the common channel [W]        Li,n is the path loss between user i and radio access port n        B is the number of radio access ports in the system        Ptot,b is the total output power for radio access port b [W]        αi is the downlink orthogonality factor for user i        Iext,j is the received external interference for user i [W]        Ni is the receiver noise floor [W]        
Now, in order to have an acceptable quality of the common channels over the whole cell area, the power of the common channels PCCH has to be tuned so that an acceptable percentile of users, i.e. between user i and radio access port n, have an SNR equal or greater than a minimum required SNR, i.e.SNRi,n≧SNRminreq assuming certain total power Ptot levels over the whole system. Thus, the required power of the common channel PCCH will be a function of the co-channel and adjacent channel interference.
Taking all the above facts into consideration, the total required output power of the radio access port depends among other things on the level of co-channel and adjacent channel interference experienced by the users connected to the radio access port in question.
In WCDMA, the handover between different radio base stations is based on measurements on the downlink Common Pilot CHannel (CPICH). A mobile terminal is defined to be at the cell border between a first cell, and a second cell, when the received CPICH downlink power is equal from a first radio access port and a second radio access port, wherein said first radio access port is located within the first cell and said second radio access port is located within the second cell. In order to balance the uplink (i.e. in the direction from the mobile terminal to the base station), the receiver sensitivity can be reduced at the radio access port with the lowest CPICH transmit power. By doing so, the received SNR can be made equal at both radio access ports. This kind of action is often referred to as desensitisation.
Usually, when a radio access port is desensitised, the receiver noise floor (Nn) is artificially raised. The required received carrier power (Ci,n) is then increased. This can be understood by solving the received carrier power from the uplink SNR equation. Thus, the required received carrier power (Ci,n) is
      C          i      ,      n        =            1              1        +                  W                                    SNR                              i                ,                n                                      ·                          R              i                                            ·          (                        I                      tot            ,            n                          +                  N          n                    )      whereItot,n=Iintra,n+Iinter,n+Iext,n 
The desensitisation can be used to balance the required uplink power at the cell border, but it has also another positive effect; it offers additional protection towards any external interference. However, it has also a few negative effects, e.g. due to the worse receiver sensitivity, the transmission power of the mobile terminals is increased, and therefore the interference towards other systems and/or cells is increased. In addition, the uplink coverage area is degraded. Therefore, it should be carefully considered when, and which amount of desensitisation should be used in each case. The architecture of a traditional radio access port 100, according to the state-of-the-art, can be illustrated as in FIG. 1a. Thus, the transmitter (Tx) and receiver (Rx) chain consist of a baseband part 102, a modem 104 and a transceiver 106. The baseband part handles data coding/decoding, encryption/removing of encryption, channel coding/decoding and interleaving/deinterleaving and the modem handles modulation (at Tx), demodulation (at Rx), channel equalizer (at Rx) and detection. In particular, the transceiver part 106 includes a High Power Amplifier (HPA) 110 for the Tx and Low Noise Amplifier (LNA) 108 for the Rx.
Typically, the maximum output power can be adjusted e.g. by adjusting the level of the input signal to the HPA. In a similar way, the receiver sensitivity can be adjusted by changing the grade of amplification in the LNA. In prior art, these two adjustments are not performed dynamically. Thus, when deploying the radio system, the levels are adjusted so that they fit to the current radio environment, but if the radio environment changes, the levels are not changed automatically.
As an example, a simple co-existence scenario between an outdoor and an indoor WCDMA system can be considered, see FIG. 2. There, each floor 202 is covered by one cell 204 consisting of one or more radio access ports 206. It is obvious, that the external interference (Iext) (or internal interference (Iinter) depending on the frequency allocation) originates from an outdoor WCDMA system is the largest on the top floors, while it has only a minor impact on the lower floors. Therefore, in order to guarantee a certain capacity throughout the building, and as low interference towards the other cells and/or systems as possible, the radio access ports 206 have to be tuned separately for each floor. Furthermore, since the interference from the outdoor base station 208, covering an outdoor cell 212, can be a major part of the total downlink interference on the top floors, the indoor capacity on those floors has a relatively strong dependency on the outdoor cell 212 loading situation.
If the uplink is considered, a great majority of the indoor-to-outdoor uplink interference is generated by the users located on the top floors. Furthermore, the outdoor-to-indoor uplink interference has the largest impact on lower floors. Therefore, the radio access port 206 sensitivity should preferably be better on higher floors compared to the lower floors. Good sensitivity results in low average transmit power for the mobile terminals 210, but a low level of protection against external uplink interference.
A disadvantage with the above-described approach is that the maximum output power level of the radio access port and the receiver sensitivity have to be manually adjusted separately for each cell in order to tune the system performance. Furthermore, the solution above may work well during some e.g. average time periods, but it does not adapt to the changes in the neighbourhood. Thus, during some e.g. heavy traffic periods, the offered capacity might be limited under the planned or required capacity, while during other e.g. light traffic periods, the transmit power levels might be greatly over-allocated, resulting in unnecessary high power consumption within the own system and interference towards other systems.
In WO01/37446 an adaptive algorithm is shown. The algorithm controls the base station transmitter gain and the base station receiver attenuation in a CDMA system in order to balance the load between the cells and thus maximize the capacity of the system. The main input to the adaptive algorithm consists of the estimated uplink noise rise (total uplink interference over the thermal noise), the so-called “F factor”. The F factor is defined as the uplink intracell interference divided by the total uplink interference (intracell interference plus intercell interference).
A method for minimizing the effect of interference in a radio system is disclosed in WO01/67634. The base station comprises means for adaptive attenuating the signal received from the mobile terminal. The goal is to maximize the attenuation, i.e. the base station should be as insensitive as possible. Power control requests or measured signal interference at the reception frequency band are used as input for the adaptive attenuation. The attenuation algorithm can also be affected if a single user experiences a too low Grade of Service. WO01/67634 discloses the features of the preamble of claim 1.
A disadvantage with WO01/67634 is that it does not control the downlink BS transmitter gain. Another disadvantage is that the BS receiver is attenuated as much as possible which results in that the mobile terminal is required to transmit with a high power level. High power transmitting terminals results further in high battery consumption which may be a problem.
Thus, the object of the present invention is to provide a method that automatic and adaptive controls the downlink transmitter gain of the base station and the uplink receiver sensitivity of the base station adaptively on a long term basis in order to minimize the impact of interference between co-existing systems.